1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus including the same, and more particularly, to a fixing device that fixes an unfixed toner image on a recording medium by applying heat and pressure thereto, and an image forming apparatus including the fixing device.
2. Description of the Background Art
Typically, a fixing device for fixing an unfixed toner image onto a recording medium is equipped with a roller-type fixing member (hereinafter referred to as fixing roller), a roller-type pressing member (hereinafter referred to as pressure roller), and a halogen heater serving as a heating member. The fixing roller is generally hollow and accommodates the halogen heater inside thereof. Supplying electric power to the halogen heater heats the fixing roller. The pressure roller is pressed against the fixing roller by an urging member.
Such a fixing device is provided with a temperature detector to detect the temperature of the surface of the fixing roller. An output of the temperature detector is provided to a temperature controller that controls the halogen heater based on the output of the temperature detector, thereby maintaining the temperature of the surface of the fixing roller at a certain temperature.
The fixing device as described above, employed typically in an electrophotographic image forming apparatus, fixes an unfixed toner image on a recording medium by applying heat and pressure to the recording medium as it passes between the fixing roller and the pressure roller, specifically, where the fixing roller and the pressure roller meet and press against each other, hereinafter called a fixing nip or simply nip.
One example of such a fixing device employed in image forming apparatuses such as copiers and printers is a so-called on-demand fixing device. A rise time of this type of fixing device is known to be short.
Various types of on-demand fixing devices have been proposed. One example of a known on-demand fixing device employs a tubular fixing film (endless film) serving as a fixing member, a pressure roller serving as a pressing member, and a heater such as a ceramic heater serving as a heating member. The heater is provided inside the tubular fixing film and pressed against an interior surface of the fixing film, thereby heating the fixing film. The fixing film contacts the pressure roller to form the nip where pressure and heat are applied to the unfixed toner image on the recording medium to fix the unfixed toner image onto the recording medium.
In order to prevent irregular fixation and paper jams in this type of fixing device, the tubular fixing film serving as the fixing member needs to be heated evenly as well as stably, and the recording medium needs to be conveyed stably. In order to do so, the fixing member needs to rotate at a constant speed.
To address such a difficulty, another example of the on-demand fixing device proposes to control the speed of rotation of the fixing film by obtaining the rpm of the fixing film using a reflective member and a reflection-type detector. An end portion of the tubular fixing film is provided with the reflective member and the reflection-type detector is provided opposite the reflection member to detect the reflective member, thereby detecting the rpm of the fixing film.
In this approach, however, due to scattered toner and undesirable adherence of toner to the surface of the reflective member over time, reflectivity of the reflective member deteriorates. As a result, the rotation speed of the fixing member cannot be detected accurately.
As described above, on-demand fixing devices employ the tubular fixing film serving as a fixing member which is pressed and heated by the heater from inside the fixing film. The fixing film and the pressure roller faun the nip where heat and pressure are applied to the unfixed toner image on the recording medium to fix the unfixed toner image onto the recording medium. Using a film member having a low heat capacity as a fixing member allows for prompt heating of the fixing member.
Although advantageous, there is a drawback in such on-demand fixing devices in that, because the heater is in constant contact with the pressing member, the pressure needs to be removed when replacing the heater or fixing a paper jam. Thus, a dedicated pressure cancellation mechanism is required. Further, because the heater is always pressed by the pressing member, the heater is easily damaged during transportation.
Still another related-art fixing device includes a fixing roller serving as a fixing member, a separation roller provided with a driving member, and a fixing belt wound around and looped between the fixing roller and the separation roller. The separation roller separates the recording medium from the fixing belt. According to this configuration, the fixing roller and the fixing belt are rotated reliably by detecting a torque of the driving member of the separation roller.
Although generally successful, the size of the fixing device tends to be large, thereby defeating the purpose of making an image forming apparatus as compact as possible.
In view of the above, a device having a simple configuration that reliably fixes an unfixed toner image for an extended period of time is required.